A construction equipment, such as an excavator, a skid steer loader and the like, includes a plurality of joints, for example, a connection part of a boom and an arm, a connection part of an arm and a bucket, and the like. The joints are configured by a pin connecting two members in a place where joint motion is generally required and a bush for reducing frictional resistance between the pin and a pin hole, and the pin and the bush are fluctuated in a lubricative environment. In addition, the joints are further configured by a housing that supports the pin and the bush, a seal for preventing contaminants, and the like.
Joints operate under various conditions depending on connection types, working conditions, and the types of joints, and in general, operate under a contact pressure of about 2 to 6 kg/mm2 and at a velocity of about 2 to 5 cm/sec. In particular, in operation an instantaneous contact pressure can reach three or more times of a general contact pressure due to an asymmetric load depending on operational conditions. Since the bush used at a low speed and under a high contact pressure requires high scuffing resistance and abrasion resistance, various compositions and forms of products have been developed.
For example, there have been provided bushes formed of a porous iron-based sintered material. In this case, a lubricant having viscosity of about 240 to 1500 cSt is impregnated in the pores of the bush. In the case of this type of bushes, a temperature increases due to frictional heat of a shaft and the inner periphery of the bush in use, and as a result the volume of the impregnated lubricant increases and the viscosity decreases so that the lubricant escapes. Meanwhile, when the bush is not operated, a capillary force increases due to the increase in the viscosity of the lubricant by cooling, and as a result the lubricant is restored into the pores. With the repetition of this reversible process, the bush was intended to maintain the lubricative state of the inner periphery. However, when the load in the lubricative region increases, the speed is reduced, or the viscosity of the lubricant decreases, the thickness of the oil film becomes small and thus the inner periphery of the bush is changed into a boundary lubrication state. Therefore, there are generated metal contact friction and adhesion due to the metal contact friction, and thus sufficient sliding effect could not be ensured under the high contact pressure.
In addition, in the related art, there have been used bushes impregnated with the lubricant to which S-based and P-based extreme pressure additives are added. However, this type of bush can only be used under a contact pressure of about 8 kg/mm2, with limited application.
In order to solve the problem, in the related art, there have also been used bushes impregnated with the lubricant to which a general extreme pressure additive and various solid lubricative particles such as graphite, MoS2, cubic BN and the like, are added.
However, in some cases, a general construction equipment does not regularly operate for several hours or more depending on the operation circumstance. When the construction equipment is not operated for several hours, the solid lubricative particles dispersed in the lubricant of the inside of the pores of the bush are precipitated and cohered. In this case, when a size of the cohered solid lubricative particles becomes several tens μm to several hundreds μm, which is a general pore size, or larger, the pores may be closed by the cohered solid lubricative particles of several tens μm to several hundreds μm or larger, or a capillary tube is closed, and the lubricant in the pores cannot escape to the inner periphery of the bush even when a temperature of the inner periphery of the bush increases after the construction equipment resumes operation. As such, an oil film is not well formed in the inner periphery of the bush and the sufficient lubricative effect is not obtained.
Instead of the bush using the lubricant having the above-mentioned problems, development of the bush using a new lubricant has been required.